Lubricant for thread rolling and thread rolling method

ABSTRACT

A thread rolling lubricant of a nonchlorine species, in which a lubricant base oil is blended with 25 through 35 weight % of (A) a sulfur species extreme pressure agent, 6 through 16 weight % of (B) an organic zinc compound, 2 through 12 weight % of (C) a calcium species additive, and 2 through 13 weight % of (D) an oiliness agent for carrying out thread rolling.

This application claims priority to Japanese patent application serial number 2007-324549, the contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a lubricant blended preferably for thread rolling and a thread rolling method using the same.

2. Description of Related Art

A screw thread can be formed by machining on a comparatively large-sized metal material of an insert nut arranged at, for example, an oil filter of an automobile or the like. However, in machining, a cutting chip is produced, and a step of removing the chip is complicated. In contrast thereto, in a case of thread rolling, a fabrication cost is low in comparison with that of machining and also the cutting chip is not produced. A strength of a rolled thread is higher than that of a cut screw. Therefore, a member forming a screw thread by machining is desired to shift to thread rolling.

When a metal material is worked, a lubricant is used. A high lubricity is mainly requested for such a lubricant. The same goes with a case of forming a screw thread by rolling. As a lubricant used for working a metal material, a chlorine species lubricant excellent in a lubricity or a seizure resistance or the like has frequently been used. However, when the chlorine species lubricant is used, a chlorine species additive component is decomposed in working or ageing which can rust a material or a tool. Also, in incinerating, a harmful substance of dioxin or the like is generated, which is adverse to an environment. Further, an incinerator is corroded or damaged to shorten a service life thereof. Therefore, there is desired a lubricant for working a metal material having a lubricity equivalent to or higher than a chlorine species lubricant. Known chlorine species lubricant having high lubricity or the like in rolling are disclosed in JP-A-5742797, JP-A-11-181458, JP-A-2001-348588, and JP-A-2006-249369.

When a screw is fabricated by rolling, a pressure exceeding a yield point of a work material is created by rotating a rolling tool of a tap, a die or the like. Then, the work material is plastically deformed, and a screw thread and a thread groove are formed. For example, when a female screw is fabricated, an inner peripheral face of a tap drill hole is plastically deformed by rotating a tap and a screw thread 1 as shown by FIG. 1 is raised. At this occasion, the screw thread 1 is raised to gather together a surrounding material along a shape of a rolling tool by a folding operation. At this occasion, as shown by FIG. 1, a top portion 2 of the screw thread 1 is frequently formed with a recess 3 produced by not gathering together the material completely. Then, a mechanical strength of the top portion 2 is weakened, and there is a concern of chipping off the top portion 2 when fastening with a male screw. Furthermore, when a debris by chipping off the top portion 2 in fastening is bit by a fitting portion of the screw (between a flank of the male screw and a flank 4 of the female screw), the following problem is posed. A fastening torque and an axial tension are unbalanced and the axial tension (fastening force) is reduced. Further, a removing operation by a seizure is difficult.

Flank angles of the female screw and the male screw are designed to be the same. However, when the recess 3 of the top portion 2 is large, there is also a danger of bringing about an error in the flank angle or a height dimension of the screw thread. The larger the recess 3, the more serious the problem.

In contrast thereto, although lubricants capable of being used also in thread rolling have been proposed in the four references, these have not been developed for thread rolling in forming a screw thread. As described above, a screw thread by screw rolling is raised by a folding operation. Therefore, as in the four references, a lubricant is not suitable for thread rolling since the lubricant is excellent simply in the lubricity. Because when the lubricant is simply excellent in the lubricity, the folding operation becomes rather difficult to be produced, and also the recess of the screw thread portion tends to be enlarged. Although when the lubricity of the lubricant is reduced, the recess of the screw thread portion can be reduced, thereby, a goal of surface roughness or shape of the screw thread is failed, and therefore, a quality of a product is deteriorated.

A raise (height) dimension and a recess dimension of the screw thread can influence on a revolution number in thread rolling. When a productivity in fabricating a screw part by thread rolling is intended to increase, it is conceivable to increase a revolution speed (revolution number) of a rolling tool. However, even when the productivity is intended to increase, the revolution number cannot simply be increased. Because with an increase in the revolution number, a work material and a rolling tool are exposed to more severe working condition. In the four references, attention is not paid particularly to a revolution number in thread rolling. Further, also a size of a recess of a screw thread top portion is not particularly investigated. Therefore, a problem remains in the lubricant of the background art for working a screw thread also in this respect.

Thus, there is a need in the art for a lubricant for thread rolling having an excellent lubricity even at a high revolution speed and capable of forming a screw thread having a small recess of a screw thread top portion and a thread rolling method using the same.

BRIEF SUMMARY OF THE INVENTION

The invention is a thread rolling lubricant of a nonchlorine species blending (A) a sulfur species extreme pressure agent, (B) an organic zinc compound, (C) a calcium species additive, and (D) an oiliness agent to a lubricant base oil as additives. When the nonchlorine species lubricant is used, the lubricant is friendly to an environment, and the life of a tool or an incinerator or the like can be increased.

Furthermore, according to a first aspect, rates of blending the respective additives by a lubricant total amount reference can be adjusted as follows. 25 through 35 weight % of the additive (A), 6 through 16 weight % of the additive (B), 2 through 12 weight % of the additive (C), and 2 through 13 weight % of the additive (D). According to a second aspect, the rates of blending the respective additives by the lubricant total amount reference can be adjusted as follows. 25 through 30 weight % of the additive (A), 6 through 11 weight % of the additive (B), 2 through 3 weight % of the additive (C), and 2 through 7 weight % of the additive (D). The lubricants according to the first and the second aspects adjusted in this way can be subjected to thread rolling at a revolution number equal to or higher than 400 rpm.

These rates of blending are particularly suitable for thread rolling. A width dimension (L) (refer to FIG. 1) of a recess of a screw thread top portion is reduced while providing an excellent lubricity even at a high thread rolling speed. In detail, in a background art, thread rolling is carried out generally by a revolution speed of about 100 through 300 rpm. In contrast thereto, according to the invention, even when thread rolling is carried out by a revolution number equal to or higher than 400 rpm, the width dimension L of the recess of the screw thread top portion can be confined to be equal to or smaller than 0.3 mm. When good lubricity is provided, also a surface roughness or the like of the screw thread is excellent, and a quality of a product is promoted. Further, according to the second aspect, the respective additives are constituted by blending amounts to be near to necessary minimum. Therefore, a material cost can be restrained while guaranteeing an excellent lubricity property. Particularly, an amount of blending the calcium species additive (C) having a comparatively high material cost is necessary minimum, and therefore, an efficiency of reducing the material cost is high.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of a screw thread.

FIG. 2 is a graph showing a relationship between a revolution number and a width dimension of a recess.

FIG. 3 is a graph showing a relationship between a revolution number and a torque.

DETAILED DESCRIPTION OF THE INVENTION Lubricant Base Oil

A lubricant base oil is not particularly limited so far as the oil is generally used for working a metal material but various lubricant base oils can be used. Representatively, there is a mineral oil or a synthetic oil. A mineral oil refined by subjecting a lubricant fraction provided by subjecting crude oil to normal pressure distillation and reduced pressure distillation to one or more of treatments of solvent bitumen removal, solvent extraction, hydrogenating decomposition, solvent dewaxing, contact dewaxing, hydrogenating refining, sulfur cleaning, clay treatment and the like can be used. As a synthetic oil, poly α-olefin, α-olefin copolymer, polybutane, alkylbenzene, polyoxyalkylene glycol, polyoxyalkylene glycol ether, silicone oil or the like is pointed out. One kind thereof may be used by itself, or two kinds or more thereof may also be mixed.

(A) Sulfur Species Extreme Pressure Agent

A sulfur species extreme pressure agent having a sulfur atom, and capable of achieving extreme pressure effect is not particularly limited so that various agents capable of achieving an equivalent effect can be used. As a sulfur species extreme pressure agent, for example, synthetic sulfur, fat sulfide, fatty acid sulfide, ester sulfide, olefin sulfide, poly sulfides, thiocarbamates, mineral oil sulfide and the like can be used. One kind thereof may be used by itself, or two kinds or more thereof may be mixed. Fat sulfide is provided by reacting sulfur and fat (lard oil, whale oil, vegetable oil, fish oil or the like). As fat sulfide, lard sulfide, rapeseed oil sulfide, castor oil sulfide, bean oil sulfide and the like can be used. As fatty acid sulfide, there is oleic acid sulfide or the like. As ester sulfide, oleic acid methyl sulfide, rice bean fatty acid octyl sulfide and the like can be used. Olefin sulfide can be provided by reacting olefin of carbon number 2 through 15 or dimer through tetramer thereof with a sulfurizing agent of sulfur chloride or the like. As polysulfides, dibenzyl polysulfide, di-tert-nonylpolysulfide, didodecylpolysulfide, di-tert-butylpolysulfide, dioctylpolysulfide, diphenylpolysulfide, dicyclohexylpolysulfide and the like can be used. As thiocarbamates, zinc dithiocarbamate, dilaurylthiodipropionate, distearyldipropionate and the like can be used. Mineral oil sulfide is constituted by dissolving single sulfur to mineral oil. Mineral oil dissolving single sulfur is not particularly restricted. For example, mineral oil lubricant base oil exemplified in the explanation of the base oil can be used.

A content of an additive (A) is constituted by 25 through 35 weight % by a total amount reference of a lubricant by pertinently maintaining a relationship with amounts of blending other additives (B) through (D), preferably, 25 through 30 weight %. When the content of the additive (A) is less than 25 weight %, the extreme pressure effect may not be achieved effectively. On the other hand, when the content of the additive (A) exceeds 35 weight %, stain, rust or the like is produced at a work, an increase in an effect compatible with the blend amount may not be achieved and a material cost is wasted. In view of cost, 25 through 26 weight % by the lubricant total reference is further preferable.

(B) Organic Zinc Compound

As organic zinc compound, zinc dialkyldithiophophate (ZnDTP), and zinc dialkyldithiocarbamate (ZnDTC) are preferable. ZNDTP is widely used in engine oil or industrial lubricant as polyfunction type additive. ZnDTP and ZnDTC are provided with chemical structures (and equivalent effects) similar to each other. ZnDTC and ZnDTP are provided with an oxidation preventive function, corrosion preventive function, load resistance function, wear preventive function and the like. Although two alkyl groups are bonded to phosphor atom by way of oxygen atom in ZNDTP, the alkyl groups may respectively be the same or may differ from each other. Although two alkyl groups are bonded to nitrogen atom in ZnDTC, the alkyl groups may respectively the same or may differ from each other. Alkyl groups of ZnDTP and ZnDTC are preferably alkyl group or aryl group of carbon number 3 or more. One kind of the additive (B) may be used by itself or two kinds or more thereof may be mixed.

A content of an additive (B) is constituted by 6 through 16 weight % by the lubricant total amount reference, preferably, 6 through 11 weight % by pertinently maintaining a relationship with amounts of blending other additives (A), (C), and (D). When the content of the additive (B) is less than 6 weight %, seizure resistance or the like may not be significantly achieved. When the content of the additive (B) exceeds 16 weight %, an increase in an effect compatible with a blend amount may not be achieved and a material cost is wasted. In view of cost, 6 through 7 weight % by the lubricant total amount reference is further preferable.

(C) Calcium Species Additive

As calcium species additive, calcium sulfonate, calcium salicylate, calcium phenate and the like can be used. These are added generally as thickeners, and excellent also in lubricity, rust resistance or the like. Although these are generally comparatively expensive, in view of price, calcium sulfonate (Ca sulfonate) is preferable. In view of lubricity, further preferably, there is highly basic calcium sulfonate having a base number equal to or larger than 250 mgKOH/g. Calcium sulfonate is a kind of sulfonates of alkali earth metal. For example, petroleum sulfonic acid provided by sulfonating aromatic component in petroleum traction component, or Ca salts of synthetic sulfonic acids of dodecylbenzene sulfonic acid, dinonylnaphthalene sulfonic acid and the like can be used. Basic calcium sulfonate is formed by adding an excess amount of calcium hydroxide to neutral calcium sulfonate and thereafter blowing carbon dioxide gas. Base number (TBN) of calcium sulfonate is further preferably equal to or larger than 350 mgKOH/g, still further preferably equal to or larger than 400 mgKOH/g. One kind of the additives (C) may be used by itself, or two kinds or more thereof may be mixed.

A content of the additive (C) is constituted by 2 through 12 weight %, preferably, 2 through 7 weight % by a lubricant total amount reference by pertinently maintaining a relationship with amounts of blending the additives (A), (B), and (D). When the content of the additive (C) is less than 2 weight %, a lubricity of a seizure resistance or the like may not be significantly be achieved. When the content of the additive (C) exceeds 12 weight %, an increase in an effect compatible with a blend amount may not be achieved, and a material cost is wasted. Further, a dynamic viscosity of a lubricant is wastefully increased, which is not preferable. In view of cost, 2 through 3 weight % thereof by the lubricant total amount reference is further preferable.

(D) Oiliness Agent

As oiliness agent, various synthetic ester compounds having ester group, fat or the like can be used. As fat, for example, beef tallow, lard, linseed oil, safflower oil, bean oil, sesame oil, corn oil, rapeseed oil, cotton seed oil, olive oil, rice bean oil, coconut oil, palm oil, palm kernel oil and the like can be used. Hydrides thereof can similarly be used. An oiliness agent mainly adjusts a lubricant, and a property of a lubricity or the like is not directly and significantly promoted by the oiliness agent per se. As ester compounds, ester compounds of synthesizing, for example, calboxylic acids of caproic acid, caprylic acid, pelargonic acid, capric acid, undecylic acid, 10-undecylewnic acid, lauric acid, tridecylic acid, myristic acid, pentadecylic acid, palmitic acid, heptadecylic acid, stearic acid, oleic acid, elaidic acid, linoleic acid, linolenic acid, stearolic acid, nanodecanic acid, arachic acid, arachidnic acid, behenic acid, cetlic acid, erucic acid, brassidic acid, lignoceric acid, cerotic acid, heptacosanic acid, montanic acid, melissic acid, lacceric acid, naphthenic acid, abietic acid, adipic acid, sebacic acid, phthalic acid, trimellitic acid, lanolic fatty acid, alkenyl succinic acid, wax oxide and various alcohols can be used.

As alcohols adopted in synthesizing ester compounds, methyl alcohol, ethyl alcohol, propyl alcohol, isopropyl alcohol, butyl alcohol, isobutyl alcohol, secbutyl alcohol, tertbutyl alcohol, n-amyl alcohol, isoamyl alcohol, hexyl alcohol, heptyl alcohol, octyl alcohol, capril alcohol, nonyl alcohol, decyl alcohol, undecyl alcohol, lauril alcohol, tridecyl alcohol, myristyl alcohol, pentadecyl alcohol, cetyl alcohol, heptadecyl alcohol, stearyl alcohol, isostearyl alcohol, oleyl alcohol, nonadecyl alcohol, eicocyl alcohol, ceryl alcohol, melicyl alcohol, neopentyl glucol, trimethylol propane, pentaerythritol, glycerin, polyalkylene glycol and the like can be used

A content of an additive (D) is constituted by 2 through 13 weight %, preferably, 2 through 7 weight % by the lubricant total amount reference by pertinently maintaining a relationship with amounts of blending other additives (A) through (C). When the content of the additive (D) is less than 2 weight %, an adjusting function of a total of the lubricant cannot be guaranteed, as a result, a lubricity may not be excellently be achieved. When the content of the additive (D) exceeds 13 weight %, an increase in an effect compatible with a blend amount may not be achieved and the material cost is wasted. In view of cost, 2 through 3 weight % by the lubricant total amount reference is further preferable.

The lubricant may be added with additives of rust preventive agent, antioxidant, anticorrosives, colorant, antifoamer, perfume and the like as necessary within a range of not hampering an effect of the invention while constituting indispensable components by additives (A) through (D). These can be blended by themselves or in mixtures thereof. However, in a case of blending the additives, when a complicated composition is constituted by adding a number of the additives, although a quality of a screw and a condition of a lubricant per se can be promoted, conversely, there is a danger of increasing the recess of the ridge top portion. Hence, it is preferable to constitute a composition to a degree capable of minimally promoting a quality of a product. Specifically, these are constituted by 2 weight % or less, preferably, 1.5 weight % or less by the lubricant total amount reference within the range of not hampering ratios of the blending additives (A) through (D). Particularly, the degree of adding the rust preventive agent is preferable. When the rust preventive agent is added, rust of a product after having been worked can be prevented from being brought about.

When a rust preventive agent is blended, a kind thereof is not particularly limited but, for example, respective sulfonates of Ba, Na, Mg and the like and sulfonic acid compounds, ester compound of wax oxide and wax oxide compounds respective salts of Ba, Na thereof, polyhydric alcohol ester such as sorbitan monolate, lanolin and metal soap of lanolin and the like can be used. Among them, Ba species rust preventive agent is preferable. According to the invention, one kind of the rust preventive agent may be used by itself or two kinds or more may be mixed. Further, the rust preventive agent are mixed with mineral oil or synthetic oil, ester or the like to facilitate to dissolve to oils.

[Thread Rolling]

In thread rolling, an inner peripheral face or an outer peripheral face of a work material is plastically deformed by using a thread rolling tool (tap or die) and raised such that a surrounding material gathers together along a shape of the rolling tool by a folding operation. When a female screw is cut at an inner peripheral face of a hole (bored previously by a drill or the like), a tap is used, and when a male screw is cut at an outer peripheral face of a rod member in a circular pillar shape, a die is used. In a case of working a female screw by a tap, the tap is rotated. In a case of working a male screw by a die, a work material is rotated. Normally, working is carried out in cold (at normal temperature). A variety of products of meter screw (M), a trapezoidal screw (TM, TW), and a taper screw (PT, PS) as well as a specially formed screw or the like can be fabricated. A principle of forming a screw thread and a recess of a top portion thereof by thread screwing has previously been explained.

A screw material constituting a work is not particularly limited so far as the screw material is basically a metal material capable of being plastically deformed such as, for example, a cold rolled steel plate or the like. A material having a high hardness and a low elongation (such as cast iron or the like) may not so suitable for rolling. When a steel material is worked, it is preferable to constitute the steel material by low carbon steel having carbon content of about 0.3 mass % or smaller. Because although when there is an increase in a carbon content of carbon steel, the hardness is increased, the carbon steel tends to be brittle. When the carbon content is low, elongation or a reduction of area is excellent. On top of that, a recess of a screw top portion can be reduced by using the lubricant precisely blended in accordance with a revolution number in thread rolling.

When a width dimension of a recess is large, a height dimension of a screw thread formed tends to be low, and therefore, an increase in a width dimension of a recess amounts to a low accuracy (quality) of a screw product. Thus, it is preferable that the recess of the screw thread portion is as small as possible to constitute a screw product having a good quality. For example, although when the width dimension of the recess is near to 0.5 mm, a significant inconvenience is not brought about basically in using the screw, but there is a case in which a failure in fastening is brought about occasionally. Although the fact does not pose a serious problem when used in a general household, but when used as a part requesting a high accuracy of (for example, an automobile part or the like), a reliability is low. In contrast thereto, when the width dimension of the recess of the screw thread top portion can be constituted by about 0.3 mm, a possibility of a failure in fastening can further be reduced.

Examples Lubricity Test

Lubricities (seizure loads) of respective lubricants and comparative examples comprising compositions shown in Table 1 through Table 4 are measured and evaluated by a four ball test. Comparative examples differ in kinds per se of additives blended. Although according to lubricant X-1, lubricant Y-1, and lubricant Z-1, respective kinds of additives blended are the same, blending rates thereof differ. Specific compositions (numerals in the table designate weight %) and a test result are shown in Table 1. Further, lubricants X-2 through X-11, lubricants Y-2 through Y-11, and lubricants Z-2 through Z-11 blending ratios of respective additives of which are variously changed are prepared by respectively constituting references by lubricant X-1, lubricant Y-1, and lubricant Z-1 and respective seizure loads are similarly measured. Compositions (numerals in table designate weight %) of lubricant X group and a test result are shown in Table 2. Compositions (numerals in table designate weight %) of lubricant Y group and a test result are shown in Table 3. Compositions (numerals in table designate weight %) of lubricant Z group and a test result are shown in Table 4. Further, although details will be described alter, as a result, lubricant Z group corresponds to the example of the invention. Dynamic viscosities of lubricant X group, lubricant Y group, and lubricant Z group in this case are respectively constituted as 20 mm²/s. The four ball test is measured based on the standard of JIS K2519. A measured material is constituted by SUJ2 both in upper balls and lower balls.

TABLE 1 Comparative Comparative Comparative Lubricant Example 1 Example 2 Example 3 X-1 Y-1 Z-1 Chlorine Chlorinated 19.5 species normal additive paraffin Chlorinated 14.6 fatty acid methyl ester Base oil Mineral oil 53.7 79.5 87.4 81.5 63.0 (A) Sulfur Synthetic 9.8 14.0 0.7 1.0 2.0 species sulfur extreme Fat sulfide 83.0 8.0 11.8 23.5 pressure mineral agent sulfide polysulfide 16.6 (B) Organic Zn-DTP 2.0 3.0 6.0 zinc composition (C) Calcium Highly basic 2.5 0.7 1.0 2.0 species Ca sulfonate additive (D) Oiliness Fat 2.0 0.7 1.0 2.0 agent Synthetic 1.4 ester Other Rust 2.4 1.0 0.5 0.8 1.5 preventive agent Total 100 100 100 100 100 100 Seizure load (kgf/cm²) 13 11 16 15 16 18

TABLE 2 Lubricant X-1 X-2 X-3 X-4 X-5 X-6 X-7 X-8 X-9 X-10 X-11 Mineral oil 87.3 83.1 78.5 83.0 78.4 83.0 78.5 83.0 78.5 82.6 77.8 (A-1) Synthetic sulfur 0.7 0.6 0.6 0.6 0.6 0.6 0.6 5.7 10.7 0.6 0.6 (A-2) Fat sulfide/mineral 8.0 7.6 7.3 7.6 7.3 7.6 7.3 7.6 7.3 13.0 18.0 sulfide (B) Zn-DTP 2.0 1.9 1.9 7.0 12.0 1.9 1.9 1.9 1.9 1.9 1.9 (C) Highly basic Ca 0.7 5.6 10.7 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6 sulfonate (D) Fat 0.7 0.6 0.6 0.6 0.6 5.7 10.7 0.6 0.6 0.6 0.6 Rust preventive agent 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 Total 100 100 100 100 100 100 100 100 100 100 100 Characteristic +C5% +C10% +B5% +B10% +D5% +D10% +A-1.5% +A-1.10% +A-2.5% +A-2.10% Seizure load (kgf/cm²) 15 16 17 15 15 15 15 15 15 16 16

TABLE 3 Lubricant Y-1 Y-2 Y-3 Y-4 Y-5 Y-6 Y-7 Y-8 Y-9 Y-10 Y-11 Mineral oil 81.5 77.3 73.1 77.2 72.9 77.3 73.1 77.3 73.1 77.4 72.1 (A-1) Synthetic sulfur 1.0 1.0 0.9 1.0 0.9 1.0 0.9 6.0 11.0 1.0 0.9 (A-2) Fat sulfide/mineral 11.8 11.2 10.7 11.2 10.7 11.2 10.7 11.2 10.7 16.2 21.8 sulfide (B) Zn-DTP 3.0 2.9 2.7 8.0 13.0 2.9 2.7 2.9 2.7 2.9 2.7 (C) Highly basic Ca 1.0 6.0 11.0 1.0 0.9 1.0 0.9 1.0 0.9 1.0 0.9 sulfonate (D) Fat 1.0 1.0 0.9 1.0 0.9 6.0 11.0 1.0 0.9 1.0 0.9 Rust preventive agent 0.8 0.7 0.7 0.7 0.7 0.7 0.7 0.7 0.7 0.7 0.7 Total 100 100 100 100 100 100 100 100 100 100 100 Characteristic +C5% +C10% +B5% +B10% +D5% +D10% +A-1.5% +A-1.10% +A-2.5% +A-2.10% Seizure load (kgf/cm²) 16 17 18 16 16 17 17 16 16 17 17

TABLE 4 Lubricant Z-1 Z-2 Z-3 Z-4 Z-5 Z-6 Z-7 Z-8 Z-9 Z-10 Z-11 Mineral oil 63.0 59.8 56.4 60.0 56.0 60.0 56.0 60.0 56.0 60.0 56.0 (A-1) Synthetic sulfur 2.0 1.9 1.8 1.9 1.8 1.9 1.8 6.7 12.2 1.9 1.8 (A-2) Fat sulfide/mineral 23.5 22.4 21.4 22.4 21.4 22.4 21.4 22.4 21.4 27.1 31.7 sulfide (B) Zn-DTP 6.0 5.7 5.5 10.5 15.8 5.7 5.5 5.7 5.5 5.7 5.5 (C) Highly basic Ca 2.0 6.9 11.8 1.9 1.8 1.9 1.8 1.9 1.8 1.9 1.8 sulfonate (D) Fat 2.0 1.9 1.8 1.9 1.8 6.7 12.2 1.9 1.8 1.9 1.8 Rust preventive agent 1.5 1.4 1.4 1.4 1.4 1.4 1.4 1.4 1.4 1.4 1.4 Total 100 100 100 100 100 100 100 100 100 100 100 Characteristic +C5% +C10% +B5% +B10% +D5% +D10% +A-1,5% +A-1,10% +A-2,5% +A-2,10% Seizure load (kgf/cm²) 18 19 19 18 18 19 19 18 18 19 19

It is known from Table 1 that according to Comparative Example 1 and Comparative Example 2 kinds of additives of which significantly differ, seizure loads are low and lubricities are not excellent. In contrast thereto, it is known that Comparative Example 3, lubricant X-1, lubricant Y-1, and lubricant Z-1 including highly basic calcium sulfonate constituting calcium species additive are provided with high seizure loads and excellent lubricities. When Comparative Example 3, lubricant X-1, lubricant Y-1, and lubricant Z-1 are compared in details, the lubricity of lubricant X-1 having a small amount of the additive as a whole is comparatively low, and Comparative Example 3 and lubricant Y-1 having substantially equivalent amounts of the additive as a whole are provided with equivalent lubricities. Further, Z-1 having a large amount of the additive as a whole is provided with the highest lubricity. Thereby, it is known that in order to guarantee an excellent lubricity, it is necessary that additive (A) is equal to or larger than 25 weight %, additive (B) is equal to or larger than 6 weight %, additive (C) is equal to or larger than 2 weight %, and additive (D) is equal to or larger than 2 weight %.

It is known from Table 2 through 4, that the additive effecting an influence on the lubricity the most is (C) calcium species additive. Thereby, it is known that a lubricity as in lubricant X group can be guaranteed by increasing the amount of (C) calcium species additive as in, for example, lubricant Y-3. However, in that case, cost becomes high. On the other hand, it is known that the lubricity as in lubricant Z group cannot be achieved even when amounts of additives (A), (B), (D) are respectively increased in lubricant X group and lubricant Y group. Similarly, even when amounts of additives (A), (B), (D) are respectively increased in lubricant Z group, a significant difference is not observed in the lubricities.

It is known from the above-described result that lubricant X group blending 25 through 35 weight % of additive (A), 6 thorough 16 weight % of additive (B), 2 through 12 weight % of additive (C), and 2 through 13 weight % of additive (D) to a lubricant base oil is preferable. That is, lubricant (z) group corresponds to the example of the invention. Further, it is known that equivalent lubricities can be guaranteed even when amounts of the respective additives are increased, and therefore, in consideration of a material cost, lubricant Z-2, lubricant Z-4, lubricant Z-6, lubricant Z-8, lubricant Z-10 having comparatively small amounts of blending additives are further preferable, and lubricant Z-1 is still further preferable. When these are taken into consideration generally, it is derived that it is further preferable to blend 25 through 30 weight % of additive (A), 6 through 11 weight % of additive (B), 2 through 7 weight % of additive (C), and 2 through 7 weight % of additive (D) to the lubricant base oil, particularly, it is still further preferable to blend 25 through 30 weight % of additive (A), 6 through 11 weight % of additive (B), 2 through 3 weight % of additive (C), and 2 through 7 weight % of additive (D) to the lubricant base oil in order to achieve sufficient cost reduction.

<Thread Rolling Test>

Next, thread rolling is carried out by using Comparative Example 3 and lubricant Z-1. Comparative Example 3 is a lubricant having the highest lubricity in comparative examples. Lubricant Z-1 is constituted by a composition constituting a reference in lubricant Z group in correspondence with the example of the invention. Further, a tendency of a torque and a width dimension L (refer to FIG. 1) of a recess of a screw thread top portion in accordance with a change in a revolution number in working a thread rolling screw is compared and investigated. FIG. 2 shows a relationship between a revolution number and a width dimension L of a recess. FIG. 3 shows a relationship between a revolution number and a torque.

[Working Condition]

Tap: tap for meter screw

-   -   diameter; φ65     -   material; substantially SKH58

Work: reinforce plate

-   -   material; SPHD     -   tensile strength; 270 N/mm²     -   thickness; 2.3 mm     -   width; 125.4 mm

In FIG. 2, the recess of the screw thread top portion is larger as a whole in a case of thread rolling by lubricant Z-1 than a case of thread rolling by Comparative Example 3. This is caused by that an absolute lubricity of lubricant Z-1 is higher than that of Comparative Example 3. However, in both of Comparative Example 3 and lubricant Z-1 the recess of the screw thread top portion tends to be small with an increase in the revolution number. It is known that even in lubricant Z-1, when the revolution number is equal to or larger than 400 rpm, the width dimension L of recess can be made to be equal to or smaller than 0.3 mm. On top of that, when FIG. 3 is investigated, it is known that also the torque tends to be increased with an increase in the revolution number of Comparative Example 3. It is known that a problem remains as the lubricant for working the thread rolling screw at high revolution thereby. Because even when the recess of the screw thread top portion can be reduced, accuracy/quality of the screw thread per se are deteriorated such that a surface roughness is failed. In contrast thereto, according to lubricant Z-1, with an increase in the revolution number, also the torque tends to be reduced, showing a behavior contrary to that of Comparative Example 3. Thereby, it is known that lubricant Z group including lubricant Z-1 is particularly suitable for working the thread rolling screw at high revolution.

Next, an influence of calcium species additive (C) effecting the influence on the lubricity the most effecting on working the thread rolling screw is confirmed. Specifically, thread rolling is carried out by using lubricant Z-1 through lubricant Z-3, and the torques and the width dimensions L of the recesses in this case are measured. A result thereof is shown in Table 5. The revolution number in this case is 400 rpm, and the working condition is similar to that of the previous thread rolling test. Further, here, lubricant Z-1 through lubricant Z-3 having dynamic viscosity of 20 mm²/s and dynamic viscosity of 40 mm²/s are used.

TABLE 5 Lubricant Z-1 Z-1 Z-2 Z-2 Z-3 Z-3 Dynamic 20 40 20 40 20 40 viscosity (mm²/s) Torque (N) 2345 2169 2199 2153 2156 2131 Width 0.30 0.29 0.32 0.30 0.30 0.29 dimension of recess (mm)

It has been confirmed already by the previously lubricity test that the larger the amount of blending calcium species additive (C), the higher the lubricity. When Table 5 is observed on this premise, even in lubricant Z-1 through lubricant Z-3 having significant differences in lubricities, in thread rolling, the torques and the sizes of the recesses of the screw thread top portions are substantially equivalent. Thereby, it is known that calcium species additive (C) is not important from a view point of promoting the quality of the screw thread in thread rolling. Therefore, it is known that a lubricant capable of forming a screw thread having a small recess of a screw thread top portion while providing an excellent lubricity and having a low material cost can be provided by restraining an amount of blending additive (C) to be a necessary minimum.

Further, it is known that the above-described effect can be ensured when the dynamic viscosity is at least about 15 through 45 mm²/s. When observed further in details, both of the torque and the recess of the screw thread top portion are reduced in a case of lubricant having a high dynamic viscosity. Therefore, it is known that the dynamic viscosity is preferably about 30 through 45 mm²/s. 

1. A lubricant for thread rolling which is a nonchlorine species lubricant, the lubricant comprising: a lubricant base oil blended with (A) a sulfur species extreme pressure agent, (B) an organic zinc compound, (C) a calcium species additive, and (D) an oiliness agent, as additive; wherein additive (A) is present in an amount of 25 through 35 weight %, additive (B) is present in the amount of 6 through 16 weight %, additive (C) is present in an amount of 2 through 12 weight %, and additive (D) is present in an amount of 2 through 13 weight %.
 2. The lubricant for thread rolling according to claim 1 a revolution of thread rolling is equal to or higher than 400 rpm.
 3. The lubricant for thread rolling according to claim 1, wherein the additive (C) is at least one of calcium sulfonate, calcium salicylate, and calcium phenate.
 4. The lubricant for thread rolling according to claim 1, wherein the additive (C) is highly basic calcium sulfonate having a base number equal to or higher than 250 mgKOH/g.
 5. The lubricant for thread rolling according to claim 1, wherein the additive (B) is at least one of ZnDTP and ZnDTC.
 6. The lubricant for thread rolling according to claim 1, further including a rust preventative agent present in an amount of 2 weight % or smaller.
 7. The lubricant for thread rolling according to claim 6, wherein the rust preventive agent is a Ba species rust preventive agent.
 8. The lubricant for thread rolling according to claim 1, wherein a dynamic viscosity is 15 through 45 mm²/s.
 9. The lubricant for thread rolling according to claim 1, wherein an object of working is low carbon steel having a carbon content equal to or smaller than 0.3 mass %.
 10. A lubricant for thread rolling which is a nonchlorine species lubricant, the lubricant comprising: a lubricant base oil blended with (A) a sulfur species extreme pressure agent, (B) an organic zinc compound, (C) a calcium species additive, and (D) an oiliness agent, as additive; wherein additive (A) is present in an amount of 25 through 30 weight %, additive (B) is present in the amount of 6 through 11 weight %, additive (C) is present in an amount of 2 through 3 weight %, and additive (D) is present in an amount of 2 through 7 weight %.
 11. The lubricant for thread rolling according to claim 10 a revolution of thread rolling is equal to or higher than 400 rpm.
 12. The lubricant for thread rolling according to claim 10, wherein the additive (C) is at least one of calcium sulfonate, calcium salicylate, and calcium phenate.
 13. The lubricant for thread rolling according to claim 10, wherein the additive (C) is highly basic calcium sulfonate having a base number equal to or higher than 250 mgKOH/g.
 14. The lubricant for thread rolling according to claim 10, wherein the additive (B) is at least one of ZnDTP and ZnDTC.
 15. The lubricant for thread rolling according to claim 10, further including a rust preventative agent present in an amount of 2 weight % or smaller.
 16. The lubricant for thread rolling according to claim 15, wherein the rust preventive agent is a Ba species rust preventive agent.
 17. The lubricant for thread rolling according to claim 10, wherein a dynamic viscosity is 15 through 45 mm²/s.
 18. The lubricant for thread rolling according to claim 10, wherein an object of working is low carbon steel having a carbon content equal to or smaller than 0.3 mass %.
 19. A thread rolling method of carrying out thread rolling by a revolution number equal to or higher than 400 rpm while supplying the lubricant according to claim
 1. 20. The thread rolling method according to claim 19, wherein a dynamic viscosity of the lubricant for thread rolling is 15 through 45 mm²/s.
 21. A thread rolling method of carrying out thread rolling by a revolution number equal to or higher than 400 rpm while supplying the lubricant according to claim
 10. 22. The thread rolling method according to claim 21, wherein a dynamic viscosity of the lubricant for thread rolling is 15 through 45 mm²/s. 